Coloured diamond

ABSTRACT

A method of producing a single crystal CVD diamond of a desired colour which includes the steps of providing single crystal CVD diamond which is coloured and heat treating the diamond under conditions suitable to produce the desired colour. Colours which may be produced are, for example, in the pink-green range.

BACKGROUND OF THE INVENTION

[0001] This invention relates to a method of producing coloured diamondand more particularly coloured single crystal chemical vapour deposition(hereinafter referred to as CVD) diamond that is suitable, for example,for ornamental purposes.

[0002] Intrinsic diamond has an indirect band gap of 5.5 eV and istransparent in the visible part of the spectrum. Introducing defects orcolour centres, as they will be called hereinafter, which haveassociated energy levels within the band gap gives the diamond acharacteristic colour that is dependent on the type and concentration ofthe colour centres. This colour can result from either absorption orphotoluminescence or some combination of these two. One example of acommon colour centre present in synthetic diamond is nitrogen which,when on a substitutional lattice site in the neutral charge state, hasan associated energy level ˜1.7 eV below the conduction band—theresulting absorption gives the diamond a characteristic yellow/browncolour.

[0003] It is well known that post-growth treatment of diamond, such asirradiation with sufficiently energetic particles or radiation(electron, neutron, gamma etc) to produce lattice defects (interstitialsand vacancies) and suitable subsequent annealing, can result in theformation of colour centres such as the nitrogen vacancy [N-V] colourcentre which can give the diamond a desirable colour (see for example EP0 615 954 A1, EP 0 326 856 A1 and the references cited therein). Furthercharacteristics and artificial production of colour centres arediscussed in detail by John Walker in the Reports on Progress inPhysics, Vol. 42 1979. The artificial production method of creatingcolour centres outlined in these reports comprises the steps of forminglattice defects in crystals by electron beam irradiation and, ifnecessary, performing annealing to cause the lattice defects to combinewith nitrogen atoms contained in the crystals. However, there arelimitations to the colours and uniformity that can be produced as aconsequence of competitive defect formation and because of the stronggrowth sector dependence associated with the concentration of defectssuch as nitrogen in diamond.

[0004] The colour of a diamond coloured by utilising a post growthcolour centre formation method is the colour of the rough diamond priorto post growth treatment combined with the effect on colour of the oneor more colour centres modified or produced during post growthtreatment. In order to obtain the ornamental value desired, and thusachieve a combination of high transparency and desirable colour, it hasbeen usual practice to use diamonds that were initially eithercolourless or light yellow. This method is therefore not readilyapplicable to brown single crystal CVD diamond.

[0005] EP 671482, U.S. Pat. No. 5,672,395 and U.S. Pat. No. 5,451,430describe methods of making polycrystalline CVD diamond more transparentusing an HPHT treatment that densities the diamond.

[0006] It is also known that the colour of brown natural diamond can bealtered by annealing at high pressures and temperatures. For example,natural type IIa diamond can be made colourless by annealing at veryhigh temperatures under stabilising pressure or it may be turned pink byannealing at rather lower temperatures, again under stabilisingpressure. Brown colour in natural diamond is believed to be associatedwith plastic deformation but the exact cause of the brown colour and howit is modified by annealing is, however, still unknown.

[0007] There are three visual attributes to colour: hue, lightness andsaturation. Hue is the attribute of colour that allows it to beclassified as red, green, blue, yellow, black or white, or a hue that isintermediate between adjacent pairs or triplets of these basic hues.

[0008] White, grey and black objects are differentiated on a lightnessscale of light to dark. Lightness is the attribute of colour that isdefined by the degree of similarity with a neutral achromatic scalestarting with white and progressing through darker levels of grey andending with black.

[0009] Saturation is the attribute of colour that is defined by thedegree of difference from an achromatic colour of the same lightness. Itis also a descriptive term corresponding to the strength of a colour.The diamond trade uses adjectives such as intense, strong and vivid todenote different degrees of saturation assessed visually. In the CIEL*a*b* colour system, saturation is the degree of departure from theneutral colour axis (defined by saturation=[(a*)²+(b*)²]^(1/2), seehereinafter). Lightness is a visual quality perceived separately fromsaturation.

[0010] Methods of depositing material such as diamond on a substrate byCVD are now well established and have been described extensively in thepatent and other literature. Where diamond is being deposited on asubstrate, the method generally involves providing a gas mixture which,on dissociation, can provide hydrogen or a halogen (e.g. F,Cl) in atomicform and C or carbon-containing radicals and other reactive species,e.g. CH_(x), CF_(x) wherein x can be 1 to 4. In addition, oxygencontaining sources may be present, as may sources for nitrogen, and forboron. Nitrogen can be introduced in the synthesis plasma in many forms;typically these are N₂, NH₃, air and N₂H₄. In many processes inert gasessuch as helium, neon or argon are also present. Thus, a typical sourcegas mixture will contain hydrocarbons C_(x)H_(y) wherein x and y caneach be 1 to 10 or halocarbons C_(x)H_(y)Hal_(z) wherein x and z caneach be 1 to 10 and y can be 0 to 10 and optionally one or more of thefollowing: CO_(x), wherein x can be 0.5 to 2, O₂, H₂, N₂, NH₃, B₂H₆ andan inert gas. Each gas may be present in its natural isotopic ratio, orthe relative isotopic ratios may be artificially controlled; for examplehydrogen may be present as deuterium or tritium, and carbon may bepresent as ¹²C or ¹³C. Dissociation of the source gas mixture is broughtabout by an energy source such as microwaves, RF (radio frequency)energy, a flame, a hot filament or jet based technique and the reactivegas species so produced are allowed to deposit onto a substrate and formdiamond.

[0011] CVD diamond may be produced on a variety of substrates. Dependingon the nature of the substrate and details of the process chemistry,polycrystalline or single crystal CVD diamond may be produced.

SUMMARY OF THE INVENTION

[0012] According to the present invention, a method of producing singlecrystal CVD diamond of a desired colour includes the steps of providingsingle crystal CVD diamond which is coloured (which sometimes in itselfis desirable), and heat treating the diamond under conditions suitableto produce the desired colour.

[0013] The single crystal CVD diamond which is used as a startingmaterial is coloured and the heat treatment is carried out undercontrolled conditions suitable to produce another and desirable colourin the diamond.

[0014] It is often possible to see more than one colour in a diamond.The dominant colour is that which, under standard lighting and viewingconditions, an observer would pick if forced to find the most accuratedescription involving only one colour. A diamond with a given dominantcolour can have a colour that is modified by a range of other coloursthat border the dominant colour in three-dimensional colour space, suchas the CIE L*a*b* colour space described hereinafter. For example, inthree-dimensional colour space, the region of pink colours is borderedby white, gray, brown, orange, purple and red colour regions. Thereforea pink diamond could, in principle, show any of these colours as amodifier to different degrees and be described appropriately as, forexample, grayish pink, brownish pink or orangish pink. In thisspecification and in the claims, where an individual colour is referredto (e.g. brown coloured diamond, green diamond) this refers to thedominant colour, and secondary colour modifiers may be present.

[0015] In general, diamonds are polished in such a way that, when viewedin the intended way, the (face-up) colour is rather different from theinherent colour of the diamond that is best seen when the stone isviewed from the side. This is partly because the facets are polished insuch a way that, for rays of light reaching the observer's eyes when thestone is viewed in the intended way, the path length within the stone isgreatly increased by one or more internal reflections. The effect ofincreased path length on the colour coordinates can be modelled in theway described hereinafter.

[0016] The colour of the single crystal CVD diamond used as startingmaterial is typically brown. Under suitable conditions of heattreatment, the brown colour can be converted into any one of a number ofdesirable colours including colourless and near colourless, andparticularly fancy colours. The term “fancy” refers to a gem tradeclassification of more saturated and more desirable colours in diamond.More particularly, the heat treatment may be such as to produce a rangeof fancy green and fancy pink colours in the diamond.

[0017] The single crystal CVD diamond may be in the form of a layer or apiece of a layer, e.g. cut as a gemstone. The invention has particularapplication to thick diamond layers, that is diamond layers having athickness of greater than 1 mm and to pieces produced from such layers.Further, the CVD diamond layer preferably has uniform crystal qualitythrough its thickness so that any desirable colour is not quenched orhidden by defects related to low crystalline quality in any region ofthe layer. It is possible with such layers or pieces of such layers toproduce a range of pink and green colours, particularly fancy pink andfancy green colours, of such a nature that they could not have beenanticipated from known natural diamond heat treated by known methods, orby known HPHT synthetic material treated by known methods. Inparticular, layers of single crystal CVD diamond in excess of 1 mm thickprovide for the production of products, for example gemstones, in whicheach of three orthogonal dimensions exceeds 1 mm.

[0018] It has been found that the single crystal CVD diamond heattreated or annealed under the conditions of the invention produce arange of desirable colours which can be defined in terms of the CIEL*a*b* colour space. More particularly, it has been found that thesingle crystal CVD diamond after heat treatment, for a 1 mm thickparallel-sided layer produced from the diamond, has a CIE Lab b*co-ordinate which lies in any one of the following ranges:

[0019] 0≦b*≦8

[0020] 0≦b*≦4

[0021] 0≦b*≦2

[0022] 0≦b*≦1

[0023] The heat treatment of the single crystal CVD diamond, asmentioned above, can lead to a diamond which is colourless or nearcolourless. The near colourless diamond can be defined in terms of theCIE L*a*b* colour space. More particularly, such a heat treated diamond,for a 1 mm thick parallel-sided layer produced from the diamond, mayhave a saturation (C*) which is less than 10 or less than 5 or less than2.

[0024] The heat treatment will vary according to the nature of theas-grown CVD diamond and the desired colour to be produced in the CVDdiamond. By way of example, it has been found that thick layers of brownsingle crystal CVD diamond or pieces cut from such layers can beannealed to a range of desirable pink to green colours at temperaturesin the range of 1600 to 1700° C. for a period of time, typically fourhours, under diamond stabilising pressure. Surprisingly, the colour ofsuch thick diamond layers or pieces cut from such layers may also bechanged to colours in the pink to green range by heat treating thelayers at temperatures in the range 1400 to 1600° C. for a period oftime, typically four hours, at a pressure in the graphite stable regionin an inert or stabilising atmosphere. An example of an inert atmosphereis argon (Ar).

[0025] In one form of the invention, the single crystal CVD diamond isproduced using a process that incorporates nitrogen to a concentrationin the solid diamond of 0.05-50 ppm. The lower limit to this range ispreferably 0.1 ppm, more preferably 0.2 ppm, and even more preferably0.3 ppm. The upper limit to this range is preferably 30 ppm, morepreferably 20 ppm, and even more preferably 10 ppm. This can be achievedusing, for example, a plasma process in which nitrogen is present in thegas phase (initially in the form of N₂, NH₃, or some other N containingmolecule). In order to achieve reproducible results and tailor the finalproduct the N in the process needs to be controlled. Typicalconcentrations in the gas phase (all nitrogen gas phase concentrationsin this specification are based on the N₂ equivalent, for example one N₂molecule is equivalent to 2 NH₃ molecules) are 0.5 ppm-500 ppm, morepreferably 1 ppm-100 ppm, and more preferably 2 ppm-30 ppm, but thoseskilled in the art will understand that the uptake of nitrogen is verysensitive to the process conditions such as temperature, pressure, andgas phase composition, so the invention is not confined to these limits.

[0026] Different isotopes of nitrogen may be used, for example ¹⁴N or¹⁵N. The effect of these different isotopes on the growth chemistry andend results is generally insignificant except in as much that anydefects of which the nitrogen forms a part may have their relatedoptical bands shifted by the difference in atomic mass. Except inexample 8, ¹⁴N has been used to derive the data presented in thisspecification, but the scope of the invention covers all isotopes of N.

[0027] Uptake of impurities such as nitrogen is also sensitive to thegrowth sector, and preferably the final layer is predominantly oressentially wholly one growth sector or type of growth sector related bysymmetry. Growth sectors such as the {100}, {111}, {110}, {111} may beused, more preferably the growth sectors {100} and {113}, and mostpreferably the {100}. The diamond may in addition contain other dopants,such as P, S, and B in low concentration although the preferred methodexcludes these.

[0028] The heat treatment (anneal) is generally carried out in thetemperature range of 1200° C.-2500° C. The lower bound to this range isgenerally set by achieving acceptable kinetic rates in the processesdesired from the annealing process, in addition to selecting theequilibrium defect concentrations towards which the kinetics areprogressing. The upper bound of this range is set by practicalconsiderations, in that there is difficulty in operating HPHT processesabove 2500° C., although annealing to form, in particular, nearcolourless diamond can be enhanced by suitable annealing above thistemperature. The lower bound to this range is preferably 1250° C., morepreferably 1300° C., and even more preferably 1400° C. The upper boundto this range is preferably 2000° C., more preferably 1900° C., and evenmore preferably 1800° C. This anneal takes place for a period of time inthe range of 3-3×10⁶ seconds. The lower bound to this range ispreferably 30 seconds, more preferably 100 seconds, and even morepreferably 300 seconds. The upper bound to this range is preferably3×10⁵ seconds, more preferably 1×10⁵ seconds, even more preferably 2×10⁴seconds and even more preferably 7×10³ seconds.

[0029] The anneal may take place under diamond stabilising pressure, ormay take place near or below atmospheric pressure, for example in aninert or stabilising atmosphere. Those skilled in the art willappreciate that there is an interdependency between these variables,longer anneal times usually being required at lower temperatures, orwhen stabilising pressure is applied at the same temperature. Thus aparticular range of temperatures may be more appropriate for aparticular range of times, and both be different according to whetherstabilising pressure is used. The upper temperature limit of annealingprocesses without diamond stabilising pressure is generally 1600° C.,particularly where the anneal times are long or the process is notcarefully controlled, because of the problem of graphitisation. However,annealing up to 1800° C., and in extreme cases 1900° C. can be achievedwithout diamond stabilising pressure.

[0030] For the purposes of this specification, the pressure domain canbe considered to be split into two domains, the diamond stable region,often referred to as diamond stabilising pressure, and the graphitestable region. The most easily accessible region of the graphite stableregion is that region around atmospheric pressure (1.01×10⁵ Pa),although in a controlled gas environment is it generally fairly simpleto achieve lower pressures, e.g. 1×10² Pa-1×10⁵ Pa, and also higherpressures e.g. 1.02×10⁵ Pa-5×10⁵ Pa. The range of pressures below 5×10⁵Pa have no discernable effect on the defect annealing within the volumeof the diamond. It is further generally understood that pressures in therange of 5×10⁵ Pa to up to diamond stabilising pressures results in nobehaviour of individual defects which differs in basic nature from thatobtainable from either annealing in the diamond stable region or nearatmospheric pressure, although reaction rates for example may vary assome smooth function of the pressure between these two extremes, andtherefore the balance and interaction between defects may vary to somedegree. The annealing in the method of the invention in the graphitestable region has generally been completed at atmospheric pressure forsimplicity, but this does not limit the method of the invention incovering annealing at other pressures in the graphite stable region.

[0031] Conventionally pressures used in high pressure presses are givenin kilobar. For consistency, all pressures in this specification aregiven in Pa, with selected higher pressures converted to bar or kilobarusing the conversion factor 1 bar=1.0×10⁵ Pa.

[0032] The coloured CVD diamond crystal of the invention preferably hasa desirable hue. The hue angle for a particular hue can be found byextending the line back from the point representing that hue on the a*b*colour plot as described more fully hereinafter, and shown on FIG. 4.The hue angle of the CVD diamond after heat treatment will typically beless than 65° or less than 60° or less than 55° or less than 50°. It iswell known that pink diamonds are much admired and highly prized byjewellers, collectors and consumers because of their universallyacknowledged great beauty and rarity (Pink Diamonds, John M. King etal., Gems and Gemology, Summer 2002). In the diamond industry greendiamonds are also highly valued and greatly admired (Collecting andClassifying Coloured Diamonds, Stephen C. Hofer, 1998, Ashland PressInc. New York). In general, pink and green diamonds are more prized thepurer the colour and the weaker the influence of secondary colourmodifiers. Heat treatment or annealing conditions of the invention canincrease the purity of the colour by increasing, removing, reducing ormodifying absorption that contributes to colour modification. At thesame time, annealing or heat treatment can increase the lightness byreducing the concentration of defects that reduce absorption over wideregions of the spectrum.

[0033] Some of the colour centres that contribute to the colour of brownCVD diamond are unique to single crystal CVD diamond or pieces cut orproduced from layers of single crystal CVD diamond, and may particularlyaffect the perceived colour of thick layers. The fact that they aredifferent from those contributing to the colour of natural diamond isclear because they cause absorption bands that are not found in theabsorption spectra of natural diamond. It is believed that some of thecolour centres relate to very localised disruption of the diamondbonding within the single crystal CVD diamond. Evidence for this comesfrom Raman scattering from non-diamond carbon observable with aninfrared excitation source (eg 785 nm or 1064 nm). Such Raman scatteringis not observed for brown natural diamond. The relative strengths of theabsorption bands in the visible region of the spectrum of brown singlecrystal CVD diamond can be altered by annealing, with concurrent changesin the Raman spectrum. Changes in the absorption spectrum are observedat much lower temperatures than are required to alter the colour ofbrown natural diamond. Significant colour changes can even be achievedby annealing at atmospheric pressure in an inert atmosphere at wellbelow the temperature at which diamond graphitises in the absence ofoxygen, for example at 1600° C. or less. This was not anticipatedbecause conversion of non-diamond carbon to diamond usually requirestreatment under high pressure and temperature conditions in adiamond-stable regime.

[0034] Characteristics associated with the CVD diamond growth mechanismcan result in absorption bands centred at about 350 nm and about 510 nmand a band, centred in the near-infrared, that extends into the redregion of the visible spectrum. The colour centres responsible for thesebands therefore have an important influence on the colour of theas-grown CVD diamond. They are not present in natural or other syntheticdiamond. Gemstones polished from as-grown CVD diamond can have desirablecolours including orange brown and pinkish brown. When such diamond isheat treated or annealed under the conditions of the invention therelative strengths of the absorption bands can be altered, e.g. removedor reduced or increased, in a way that enhances the colour. Acontribution to the colour change may also come from formation of colourcentres via the breaking up of defects existing in the as-grown diamondor from changes in charge transfer processes that alter the dominantcharge state of defects. The annealing or heat treatment step cantherefore alter the combinations of colour centres to combinations thatcannot be produced in as-grown CVD diamond, giving single crystal CVDdiamond that has desirable colour coming from a novel combinations ofcolour centres. As known by persons skilled in the art, broad bands suchas the 350 nm and 510 nm may exhibit small variations in the position ofmaximum intensity, but this does not change their identity.

BRIEF DESCRIPTION OF THE DRAWINGS

[0035]FIG. 1 The UV-visible absorption spectra of sample Ex-4 recorded(a) before and (b) after annealing at 2400° C. for 4 hours at a pressureof approximately 8.0×10⁹ Pa (80 kbar)

[0036]FIG. 2 The UV-visible absorption spectra of sample Ex-5 recorded(a) before and (b) after annealing at 1900° C. for 4 hours at a pressureof approximately 7.0×10⁹ Pa (70 kbar)

[0037]FIG. 3 The UV-visible absorption spectra of sample Ex-6 recorded(a) before and (b) after annealing at 1600° C. for 4 hours at a pressureof approximately 6.5×10⁹ Pa (65 kbar)

[0038]FIG. 4A plot of the CIELAB a* and b* values derived from theUV/visible absorption spectra of Ex-6 recorded (a) before and (b) afterannealing at 1600° C. for 4 hours at a pressure of approximately 6.5×10⁹Pa (65 kbar)

[0039]FIG. 5A plot of the CIELAB L* and C* values derived from theUV/visible absorption spectra of Ex-6 recorded (a) before and (b) afterannealing at 1600° C. for 4 hours at a pressure of approximately 6.5×10⁹Pa (65 kbar)

DESCRIPTION OF EMBODIMENTS

[0040] The invention achieves the controlled conversion of a colouredsingle crystal CVD diamond to another colour under suitable andcontrolled heat treatment conditions. The single crystal CVD diamond ispreferably in the form of a thick layer or a piece cut or produced fromsuch a layer. The thick layer of single crystal CVD diamond should be ofhigh quality and is preferably made by providing a diamond substratehaving a surface which is substantially free of crystal defects,providing a source gas, dissociating the source gas and allowinghomo-epitaxial diamond growth on the surface which is substantially freeof crystal defects. CVD diamond grown in such a manner is free of theinclusions which typify HPHT diamond, particularly HPHT diamond wherethe colour is not dominated by single substitutional nitrogen.

[0041] Generally, the method is carried out in the presence of nitrogenwhich is added to the synthesis plasma. The presence of nitrogenproduces brown colour centres in the diamond. When added in a controlledway, nitrogen disrupts the growth of the diamond sufficiently to causethe incorporation of colour centres involving carbon bonded innon-diamond ways, while giving diamond that is of good single crystalquality as judged using x-ray techniques such as X-ray topography.

[0042] It is important for the production of high crystalline qualitythick single crystal CVD diamond that growth takes place on a diamondsurface which is substantially free of crystal defects. In this context,defects primarily mean dislocations and micro cracks, but also includetwin boundaries, point defects not intrinsically associated with thedopant N atoms, low angle boundaries and any other extended disruptionto the crystal lattice. Preferably the substrate is a low birefringencetype Ia natural, Ib or IIa high pressure/high temperature syntheticdiamond or a CVD synthesised single crystal diamond.

[0043] The quality of growth on a substrate which is not sufficientlyfree of defects rapidly degrades as the layer grows thicker and as thedefect structures multiply, causing general crystal degradation,twinning and renucleation.

[0044] The defect density is most easily characterised by opticalevaluation after using a plasma or chemical etch optimised to reveal thedefects (referred to as a revealing plasma etch), using for example abrief plasma etch of the type described below. Two types of defects canbe revealed:

[0045] 1) Those intrinsic to the substrate material quality. In selectednatural diamond the density of these defects can be as low as 50/mm²with more typical values being 10²/mm², whilst in others it can be10⁶/mm² or greater.

[0046] 2) Those resulting from polishing, including dislocationstructures and microcracks forming chatter tracks along polishing lines.

[0047] The density of these can vary considerably over a sample, withtypical values ranging from about 10²/mm², up to more than 10⁴/mm² inpoorly polished regions or samples.

[0048] The preferred low density of defects is such that the density ofsurface etch features related to defects, as described above, are below5×10³/mm², and more preferably below 10²/mm².

[0049] The defect level at and below the substrate surface on which theCVD growth takes place may thus be minimised by careful preparation ofthe substrate. Included here under preparation is any process applied tothe material from mine recovery (in the case of natural diamond) orsynthesis (in the case of synthetic material) as each stage caninfluence the defect density within the material at the plane which willultimately form the substrate surface when preparation as a substrate iscomplete. Particular processing steps may include conventional diamondprocesses such as mechanical sawing, lapping and polishing (in thisapplication specifically optimised for low defect levels), and lessconventional techniques such as laser processing or ion implantation andlift off techniques, chemical/mechanical polishing, and both liquid andplasma chemical processing techniques. In addition, the surface R_(Q)(root mean square deviation of surface profile from flat measured bystylus profilometer, preferably measured over 0,08 mm length) should beminimised, typical values prior to any plasma etch being no more than afew nanometers, i.e. less than 10 nanometers.

[0050] One specific method of minimising the surface damage of thesubstrate, is to include an in situ plasma etch on the surface on whichthe homoepitaxial diamond growth is to occur. In principle this etchneed not be in situ, nor immediately prior to the growth process, butthe greatest benefit is achieved if it is in situ, because it avoids anyrisk of further physical damage or chemical contamination. An in situetch is also generally most convenient when the growth process is alsoplasma based. The plasma etch can use similar conditions to thedeposition or diamond growing process, but with the absence of anycarbon containing source gas and generally at a slightly lowertemperature to give better control of the etch rate. For example, it canconsist of one or more of:

[0051] (i) an oxygen etch using predominantly hydrogen with optionally asmall amount of Ar and a required small amount of O₂. Typical oxygenetch conditions are pressures of 50-450×10² Pa, an etching gascontaining an oxygen content of 1 to 4 percent, an argon content of 0 to30 percent and the balance hydrogen, all percentages being by volume,with a substrate temperature 600-1100° C. (more typically 800° C.) and atypical duration of 3-60 minutes.

[0052] (ii) a hydrogen etch which is similar to (i) but where the oxygenis absent.

[0053] (iii) alternative methods for the etch not solely based on argon,hydrogen and oxygen may be used, for example, those utilising halogens,other inert gases or nitrogen.

[0054] Typically the etch consists of an oxygen etch followed by ahydrogen etch and then moving directly into synthesis by theintroduction of the carbon source gas. The etch time/temperature isselected to enable remaining surface damage from processing to beremoved, and for any surface contaminants to be removed, but withoutforming a highly roughened surface and without etching extensively alongextended defects such as dislocations which intersect the surface andthus cause deep pits. As the etch is aggressive, it is particularlyimportant for this stage that the chamber design and material selectionfor its components be such that no material is transferred by the plasmainto the gas phase or to the substrate surface. The hydrogen etchfollowing the oxygen etch is less specific to crystal defects roundingoff the angularities caused by the oxygen etch which aggressivelyattacks such defects and providing a smoother, better surface forsubsequent growth.

[0055] The surface or surfaces of the diamond substrate on which the CVDdiamond growth occurs are preferably the {100}, {110}, {113} or {111}surfaces. Due to processing constraints, the actual sample surfaceorientation can differ from these ideal orientations up to 5°, and insome cases up to 10°, although this is less desirable as it adverselyaffects reproducibility.

[0056] It is also important in the method of the invention that theimpurity content of the environment in which the CVD growth takes placeis properly controlled. More particularly, the diamond growth must takeplace in the presence of an atmosphere containing substantially nocontaminants other than the intentionally added nitrogen or otherdopants. The nitrogen concentration should be controlled to better than500 parts per billion (as a molecular fraction of the total gas volume)or 5% of the target concentration in the gas phase, whichever is thelarger, and preferably to better than 300 parts per billion (as amolecular fraction of the total gas volume) or 3% of the targetconcentration in the gas phase, whichever is the larger, and morepreferably to better than 100 parts per billion (as a molecular fractionof the total gas volume) or 1% of the target concentration in the gasphase, whichever is the larger. Measurement of absolute and relativenitrogen concentration in the gas phase at concentrations as low as 100ppb requires sophisticated monitoring equipment such as that which canbe achieved, for example, by gas chromatography. An example of such amethod is now described:

[0057] Standard gas chromatography (GC) art consists of: a gas samplestream is extracted from the point of interest using a narrow boresample line, optimised for maximum flow velocity and minimum deadvolume, and passed through the GC sample coil before being passed towaste. The GC sample coil is a section of tube coiled up with a fixedand known volume (typically 1 cm³ for standard atmospheric pressureinjection) which can be switched from its location in the sample lineinto the carrier gas (high purity He) line feeding into the gaschromatography columns. This places a sample of gas of known volume intothe gas flow entering the column; in the art, this procedure is calledsample injection.

[0058] The injected sample is carried by the carrier gas through thefirst GC column (filled with a molecular sieve optimised for separationof simple inorganic gases) and is partially separated, but the highconcentration of primary gases (e.g. H₂, Ar) causes column saturationwhich makes complete separation of, for example nitrogen difficult. Therelevant section of the effluent from the first column is then switchedinto the feed of a second column, thereby avoiding the majority of theother gases being passed into the second column, avoiding columnsaturation and enabling complete separation of the target gas (N₂). Thisprocedure is called “heart-cutting”.

[0059] The output flow of the second column is put through a dischargeionisation detector (DID), which detects the increase in leakage currentthrough the carrier gas caused by the presence of the sample. Chemicalstructure is identified by the gas residence time which is calibratedfrom standard gas mixtures. The response of the DID is linear over morethan 5 orders of magnitude, and is calibrated by use of specialcalibrated gas mixtures containing the species to be detected, typicallyin the range of 10-100 ppm, made by gravimetric analysis and thenverified by the supplier. Linearity of the DID can be verified bycareful dilution experiments. Target N containing species include thetype of gas used as a deliberate dopant (e.g. N₂, NH₃) and also N₂ whichmay result from atmospheric contamination, and any other n containingspecies which may be relevant to the conditions of measurement.

[0060] This known art of gas chromatography has been further modifiedand developed for this application as follows: The processes beinganalysed here are typically operating at 50-500×10² Pa. Normal GCoperation uses the excess pressure over atmospheric pressure of thesource gas to drive the gas through the sample line. Here, the sample isdriven by attaching a vacuum pump at the waste end of the line and thesample drawn through at below atmospheric pressure. However, whilst thegas is flowing the line impedance can cause significant pressure drop inthe line, affecting calibration and sensitivity. Consequently, betweenthe sample coil and the vacuum pump is placed a valve which is shut fora short duration before sample injection in order to enable the pressureat the sample coil to stabilise and be measured by a pressure gauge. Toensure a sufficient mass of sample gas is injected, the sample coilvolume is enlarged to about 5 cm³. Dependent on the design of the sampleline, this technique can operate effectively down to pressures of about70×10² Pa. Calibration of the GC is dependent on the mass of sampleinjected, and the greatest accuracy is obtained by calibrating the GCusing the same sample pressure as that available from the source underanalysis. Very high standards of vacuum and gas handling practice mustbe observed to ensure that the measurements are correct.

[0061] The point of sampling may be upstream of the synthesis chamber tocharacterise the incoming gases, within the chamber to characterise thechamber environment, or downstream of the chamber.

[0062] The source gas may be any known in the art and will contain acarbon-containing material which dissociates producing radicals or otherreactive species. The gas mixture will also generally contain gasessuitable to provide hydrogen or a halogen in atomic form.

[0063] The dissociation of the source gas is preferably carried outusing microwave energy in a reactor examples of which are known in theart. However, the transfer of any impurities from the reactor should beminimised. A microwave system may be used to ensure that the plasma isplaced away from all surfaces except the substrate surface on whichdiamond growth is to occur and its mount. Examples of a preferred mountmaterials are: molybdenum, tungsten, silicon and silicon carbide.Examples of preferred reactor chamber materials are stainless steel,aluminium, copper, gold and platinum.

[0064] A high plasma power density should be used, resulting from highmicrowave power (typically 3-60 kW, for substrate diameters of 25-300mm) and high gas pressures (50-500×10² Pa, and preferably 100-450×10²Pa).

[0065] Using the above conditions it has been possible to produce thickhigh quality single crystal CVD diamond layers with a brown colour usingnitrogen additions to the gas flow in the range 0.1 to 500 ppm.

[0066] The thick high quality single crystal CVD diamond or a piecethereof is then subjected to heat treatment. The piece may, for example,take the form of a gemstone.

[0067] Embodiments of the invention will now be described. Table 1 belowlists seven different combinations (referred to as Case 1-7) ofabsorption bands that can be found in as-grown brown single crystal CVDdiamond. The breakdown of the spectrum of brown CVD diamond into theseabsorption bands is discussed in detail in WO 03/052177A1.

[0068] The feature at 270 nm is present in each case and it relates toisolated nitrogen impurities at substitutional sites within the diamondlattice. It is well known that the associated absorption spreads intothe visible region of the absorption spectrum and gives the distinctiveyellow hue of type Ib diamond.

[0069] Case 1: The ramp feature denotes a general rise in absorptionfrom the red to ultra-violet. This is observed in the spectrum of manysingle crystal CVD diamond layers and is undesirable in that by itself,or in combination with isolated substitutional nitrogen, it gives a dullbrown hue.

[0070] Case 2: Broad bands at approximately 350 nm and 510 are believedto be associated with localised disruption of the CVD diamond structurethat gives states within the band gap. In as-grown samples of singlecrystal CVD diamond, these features tend to appear together. Inassociation with isolated substitutional nitrogen, they can give huesranging from orange brown to pink brown, depending on the relativestrengths of the three contributions.

[0071] Case 3: Brown single crystal CVD diamond may also show a broadband centred in the NIR region of the spectrum and, when reasonablystrong, the short wavelength side of this band can give rise tosignificant absorption at the red end of the visible spectrum. By itselfthis would cause a blue hue. When seen together with isolatedsubstitutional nitrogen the resulting absorption at both ends of thespectrum gives the diamond a green hue.

[0072] Case 4: The spectrum of many brown single crystal CVD diamondlayers can be reconstructed as the sum of isolated substitutionalnitrogen, ramp and 350/510 nm band contributions. This combination tendsto give an orange brown hue.

[0073] Cases 5, 6 and 7 cover three other combinations of the absorptionfeatures discussed above. These combinations can give a range ofdifferent brown hues depending on the relative strengths of thecomponent absorption features. TABLE 1 NIR 270 nm Ramp 350 nm 510 nmband Resultant colour Case 1 Yes Yes No No No Dull brown Case 2 Yes NoYes Yes No Pinkish brown Case 3 Yes No No No Yes Green Case 4 Yes YesYes Yes No Orange Brown Case 5 Yes Yes No No Yes Brown Case 6 Yes No YesYes Yes Brown Case 7 Yes Yes Yes Yes Yes Brown

[0074] Brown single crystal CVD diamond was annealed under variousconditions and the effect thereof was observed.

[0075] It was found that the ramp and the 350 nm band can besubstantially removed by treatment at 1400-1600° C. for four hours atatmospheric pressure in an inert atmosphere. A similar effect can alsobe achieved by annealing at 1600-1700° C. for four hours underdiamond-stabilising pressure. By itself these treatments can have asignificant effect on the colour of the diamond in, for example, cases1, 2, 4, 5, 6 and 7 as shown in table 2. TABLE 2 Initial colour Finalcolour Case 1 Dull brown Lighter brown/near colourless Case 2 Pinkishbrown Pink brown to brownish pink Case 4 Orange brown Pink brown tobrownish pink Case 5 Brown Green Case 6 Brown Lighter brown Case 7 BrownLighter brown

[0076] In this temperature range it is also possible for absorptionrelating to the negatively charged nitrogen-vacancy centre (with azero-phonon line at 637 nm) to be significantly increased. The increasein the associated absorption (peaking at approximately 550 nm) tends tomake samples look pinker in colour. This increased absorption may becaused by a change in charge transfer that causes more of thenitrogen-vacancy centres to be in the negative charge state. It may becaused by the formation of additional nitrogen-vacancy centres as aresult of capture of released vacancies at isolated nitrogen ordissociation of more complex defects. There may also be an increase inthe luminescence excited from negative nitrogen vacancy centres and inextreme cases this could affect the apparent colour of the diamond.

[0077] Thus, by choosing suitable conditions of heat treatment, it ispossible to produce coloured single crystal CVD diamond having a fancycolour ranging from pink to green.

[0078] It was found that annealing for four hours at 1800° C. underdiamond stable pressure conditions causes a slight decrease in both the510 nm band and the NIR band, giving the colour changes shown in table 3in the cases considered. TABLE 3 Initial colour Final colour Case 1 Dullbrown Near colourless Case 2 Pinkish brown Pink brown to brownish pinkCase 3 Green Less intense green Case 4 Orange brown Pinkish Case 5 BrownGreenish Case 6 Brown Lighter brown Case 7 Brown Lighter brown

[0079] It was found that annealing for four hours at 1900° C. or athigher temperatures under diamond stabilising pressure removes both the510 nm band and the NIR band, giving the colour changes shown in table 4in the cases considered. TABLE 4 Initial colour Final colour Case 1 Dullbrown Near colourless Case 2 Pinkish brown Near colourless Case 3 GreenNear colourless Case 4 Orange brown Near colourless Case 5 Brown Nearcolourless Case 6 Brown Near colourless Case 7 Brown Near colourless

[0080] Under these annealing conditions the nitrogen-vacancy centre canbe dissociated into an isolated substitutional nitrogen and a vacancythat migrates away. Nitrogen vacancy centres are therefore less likelyto influence the colour of (formerly brown) diamond after annealingtreatments at this or higher temperatures. After such treatments suchdiamond does however show strong green luminescence that may give it agreenish hue under some viewing and lighting conditions.

[0081] When excited with 325 nm HeCd laser light, the photoluminescencespectra of brown CVD diamond that has been annealed at temperatures highenough to substantially dissociate the nitrogen-vacancy defects formedduring the growth process, tends to be dominated by bands in thespectral region between 450 and 550 nm. H3 luminescence (with azero-phonon line at 503 nm) may be observed and, after anneals at thehighest temperatures, N3 luminescence (with a zero-phonon line at 415nm) may also be detected. With above bandgap UV or electron beamexcitation, there is a tendency for the dominant visible luminescence tochange from green to blue as the annealing time is increased for thehighest temperature anneals.

[0082] There are other photoluminescence lines in the spectrum of(formerly brown) single crystal CVD diamond that has been annealed. Forexample, a photoluminescence line at approximately 851 nm is readilyexcited with a 785 nm laser. Although not shown by as-grown brown CVDdiamond, this line has been detected for brown CVD diamond annealed attemperatures as low as 1200° C. This photoluminescence line has neverbeen seen in any other kind of diamond and therefore appears to beunique to brown CVD diamond that has been annealed under conditions thatcould change its colour.

[0083] Using Nd:YAG laser excitation (1064 nm) other photoluminescencelines may be observed at 1263 nm, 1274 nm and 1281 nm. These have alsoonly been observed for single crystal brown CVD diamond that has beenannealed under conditions that could change its colour.

[0084] As-grown brown single crystal CVD diamond that can have itscolour significantly improved by annealing treatments may show infraredabsorption bands relating to the stretch modes of carbon-hydrogen bondsin the 2800-3000 cm⁻¹ region of the spectrum. These bands are generallyaltered but not completely removed by high temperature annealingtreatments and are not generally seen in the absorption spectra ofnatural or HPHT synthetic diamond.

[0085] Some natural diamond shows a hydrogen-related absorption line at3107 cm⁻¹ that has never been seen in the spectra of untreated CVDdiamond. Annealing of brown single crystal CVD diamond at temperaturesgreater than approximately 1800° C. can cause the formation of theH-related defect responsible for the line at 3107 cm⁻¹. This defect isextremely stable and is observed in the spectra of samples annealed atextremely high temperatures. Thus the observation of the 3107 cm−1 linein material known to be CVD is indicative that the material has beenannealed according to the method of this invention. Further, theobservation of the 3107 cm⁻¹ absorption line in combination with theCH-stretch features is particularly indicative that the diamond is CVDdiamond that has been given a post-growth high temperature annealingtreatment of a kind that has changed its colour in the way describedherein.

[0086] Optical characteristics such as those described above havebenefit in providing evidence of the prior history of a sample ofdiamond, in addition to any modification in colour that they maygenerate. Determination of the presence or absence of the opticalfeatures presented in this specification is understood by those skilledin the art.

[0087] For polycrystalline CVD diamond, voids may give rise to reducedoptical transmission towards shorter wavelengths. The single crystal CVDdiamond of the present invention does not contain voids, either beforeor after annealing. Samples of such diamond were studied closely, bothin cross-section and plan view, with a high magnification (×1000)optical microscope. Nothing that could have been a void was observed.Optical microscopy therefore sets an upper limit on the dimensions ofvoids of the order of 200 nm.

[0088] Transmission electron microscopy (TEM) allows thin slices ofdiamond to be viewed with sub-nanometre resolution. Several TEM slicesof unannealed brown CVD diamond have been imaged using TEM in order toinvestigate the possibility that the colour is caused by the presence ofextended defects. In order to generate the uniform colour, such defectswould need to be well distributed through the diamond and of significantdensity, and thus detectable by using TEM. As such this distribution isquite distinct from the distribution of dislocations or dislocationbundles observed by techniques such as X-ray topography, wheredislocations thread in the direction of growth and originate from eithersubstrate defects or from particles or other surface defects on thesubstrate used for the CVD growth. Imaging of several hundred squaremicrons of unannealed brown CVD diamond has not revealed anything thatcould correspond to a void. Only in specimens of extremely dark browndiamond were any extended defects in the form of dislocations andstacking fault defects seen at all. In the case of weak and moderatelybrown samples such extended defects were not observed at all in the areainvestigated by TEM.

[0089] The chromaticity co-ordinates can be used as a measure or meansof illustrating the difference between the fancy colours of singlecrystal CVD diamond produced by the method of the invention comparedwith those occurring in the other types or forms of diamond.

[0090] It is the perceived colour that is important in the annealed orheat treated single crystal CVD diamond of this invention and becausechromaticity coordinates relate more directly to perceived colour than atransmittance spectrum does, the use of chromaticity coordinates assistsin showing the novel features of such diamond. Differences in theabsorption spectrum of the CVD diamond of this invention give rise toperceived colours that can be different from those previouslydemonstrated for other CVD diamond or HPHT synthetic diamond.

[0091] It may be true that beauty is in the eye of the beholder and thathue may be a matter of personal preference. On the other hand, it isacknowledged by the diamond industry that pink and green diamonds aremore highly prized than brown diamond and become even more prized as theinfluence of colour modifiers is reduced. Pink or green diamond is morelikely to yield a desirable colour in a skillfully polished stone if ithas a high lightness for a given saturation. A polished diamond with thesame hue and saturation is less likely to give a pleasing colour if ithas low lightness.

[0092] Furthermore, there are applications in optics and electromagnetictransmission where for example a window is required to have certainabsorption characteristics. This may simply be a low overall absorption,or low absorption in certain bands, or it may include the need forspecific absorption peaks such as for applications measuring radiationby calorimetric means. Thus the diamond of the invention has particularuse in optical applications. Optical applications may not be restrictedto the visible region, but may extend into the UV and into the IR andbeyond. In particular it is anticipated that this material will alsohave application in the microwave region.

[0093] CIE L*a*b* Chromaticity Coordinate Derivation

[0094] The perceived colour of an object depends on thetransmittance/absorbance spectrum of the object, the spectral powerdistribution of the illumination source and the response curves of theobserver's eyes. The CIELAB chromaticity coordinates quoted in thispatent application have been derived in the way described below. Using astandard D65 illumination spectrum and standard (red, green and blue)response curves of the eye (G. Wyszecki and W. S. Stiles, John Wiley,New York-London-Sydney, 1967) CIE L*a*b* chromaticity coordinates of aparallel-sided plate of diamond have been derived from its transmittancespectrum using the relationships below, between 350 nm and 800 nm with adata interval of 1 nm:

[0095] S_(λ)=transmittance at wavelength λ

[0096] L_(λ)=spectral power distribution of the illumination

[0097] x_(λ)=red response function of the eye

[0098] y_(λ)=green response function of the eye

[0099] z_(λ)=blue response function of the eye

X=Σ _(λ) [S _(λ) x _(λ) L _(λ) ]/Y ₀

Y=Σ _(λ) [S _(λ) y _(λ) L _(λ) ]/Y ₀

Z=Σ _(λ[S) _(λ) z _(λ) L _(λ) ]/Y ₀

[0100] Where Y₀=Σ_(λ)y_(λ)y_(λ)L_(λ)

L*=116 (Y/Y ₀)^(1/3)−16=Lightness (for Y/Y₀>0.008856)

a*=500[(X/X ₀)^(1/3)−(Y/Y ₀)^(1/3)](for X/X₀>0.008856, Y/Y₀>0.00

b*=200[(Y/Y ₀)^(1/3)−((Z/Z ₀)^(1/3)](for Z/Z₀>0.008856)

C*=(a* ² +b* ²)^(1/2)=saturation

h _(ab)=arctan (b*/a*)=hue angle

[0101] Modified versions of these equations must be used outside thelimits of Y/Y₀, X/X₀ and Z/Z₀. The modified versions are given in atechnical report prepared by the Commission Internationale deL'Eclairage (Colorimetry (1986)).

[0102] It is normal to plot a* and b* coordinates on a graph with a*corresponding to the x axis and b* corresponding to the y axis. Positivea* and b* values correspond respectively to red and yellow components tothe hue. Negative a* and b* values correspond respectively to green andblue components. The positive quadrant of the graph then covers huesranging from yellow through orange to red, with saturations (C*) givenby the distance from the origin.

[0103] It is possible to predict how the a*b* coordinates of diamondwith a given absorption coefficient spectrum will change as the opticalpath length is varied. In order to do this, the reflection loss mustfirst be subtracted from the measured absorbance spectrum. Theabsorbance is then scaled to allow for a different path length and thenthe reflection loss is added back on. The absorbance spectrum can thenbe converted to a transmittance spectrum which is used to derive theCIELAB coordinates for the new thickness. In this way the dependence ofthe hue, saturation and lightness on optical path length can be modelledto give an understanding of how the colour of diamond with givenabsorption properties per unit thickness will depend on the optical pathlength.

[0104] L*, the lightness, forms the third dimension of the CIELAB colourspace. It is important to understand the way in which the lightness andsaturation vary as the optical path length is changed for diamond withparticular optical absorption properties. This can be illustrated on acolour tone diagram in which L* is plotted along the y-axis and C* isplotted along the x-axis (such as FIG. 4). The method described in thepreceding paragraph can also be used to predict how the L*C* coordinatesof diamond with a given absorption coefficient spectrum depend on theoptical path length.

[0105] A broad scale of lightness can be defined in the following way:Light: 95>L*>65, Medium: 65>L*>35, Dark: 35>L*>05.

[0106] The C* (saturation) numbers can be divided into saturation rangesof 10 C* units and assigned descriptive terms as below.  0-10 weak 10-20weak-moderate 20-30 moderate 30-40 moderate-strong 40-50 strong 50-60strong-very strong 60-70 very strong   70-80+ very very strong

EXAMPLE 1

[0107] A 3.2 mm thick CVD layer was grown on an HPHT synthetic diamondsubstrate. The surface of the substrate on which growth was to takeplace was prepared according to the method described in WO 01/96634.

[0108] This substrate was mounted on a tungsten substrate using a hightemperature braze suitable for diamond. This was introduced into amicrowave reactor, an etch and growth cycle used to prepare thesubstrate surface, and then growth commenced. More particularly:

[0109] 1) The reactor was pre-fitted with point of use purifiers,reducing nitrogen levels in the incoming gas stream (excluding the N₂dopant line) to below 80 ppb, as determined by the modified GC methoddescribed above.

[0110] 2) An in situ oxygen plasma etch was performed using 30/150/1200sccm (standard cubic centimetre per second) of O₂/Ar/H₂ at 235×10² Paand a substrate temperature of 840° C.

[0111] 3) This moved without interruption into a hydrogen etch at 850°C. with the removal of the O₂ from the gas flow.

[0112] 4) This moved into the growth process by the addition of thecarbon source which in this instance was CH₄ at 32 sccm. The growthtemperature at this stage was 890° C.

[0113] 5) Nitrogen (N₂) was introduced into the growth process at aconcentration of 10 ppm.

[0114] 6) On completion of the growth period, the substrate was removedfrom the reactor and the CVD layer was released from the substrate.

[0115] The single substitutional nitrogen concentration in this layerwas estimated to be approximately 0.40 ppm from the 270 nm absorptionfeature in the absorption spectrum. The absorption spectrum alsocontained broad bands centred at approximately 360 nm and 520 nm and ageneral rise (ramp) in absorption coefficient from the red to theultra-violet.

[0116] The layer was polished into a round brilliant cut stone of 0.55carats and was graded as fancy light brown, VS1. It was then annealed at1700° C. for four hours under diamond stabilising pressure ofapproximately 6.5×10⁹ Pa (65 kbar). Without any further processing, itwas then graded as fancy light pink brown, VS1.

[0117] The culet of the round brilliant was enlarged to allow aquantitative absorption spectrum to be recorded. This indicated nosignificant change in the concentration of single substitutionalnitrogen. The strength of the band at 360 nm and the ramp in theabsorption spectrum had been substantially reduced but the band atapproximately 520 nm remained largely unchanged.

[0118] An absorption spectrum recorded at 77 K showed a weak line at 637nm (with the associated vibronic bands) from negative nitrogen-vacancycentres. The photoluminescence spectrum was dominated by luminescencefrom the nitrogen-vacancy defects with zero-phonon lines at 575 nm and637 nm. Raman normalised photoluminescence spectra recorded at 77 K with514 nm excitation before and after annealing indicated that theannealing treatment had caused an increase in photoluminescence fromnitrogen-vacancy centres and this coupled with an increase in theassociated absorption may have contributed to the change in perceivedcolour.

EXAMPLE 2

[0119] A 3.1 mm thick CVD layer was grown on an HPHT synthetic diamondsubstrate using a process similar to that set out in Example 1. Thesingle substitutional nitrogen concentration in this sample wasestimated to be approximately 0.5 ppm from the strength of the 270 nmfeature in the absorption spectrum.

[0120] The layer was polished into a round brilliant of 0.49 carats thatwas graded as light brown, VS1. It was then annealed at 2100° C. fortwenty-four hours under diamond stabilising pressure of approximately7.5×10⁹ Pa (75 kbar). After repolishing to 0.44 carats, it was thengraded as fancy light greyish green, VS1.

[0121] The culet of the round brilliant was then enlarged to allow anabsorption spectrum to be recorded. By itself, the absorption spectrumwas insufficient to explain the green hue of the stone.Photoluminescence spectra (with a HeCd laser or Xe lamp excitation)showed strong green luminescence from defects (H3 and other unidentifieddefects) that were formed by the annealing process. In this case theperceived green hue is predominantly a result of the greenphotoluminescence and its dependence on viewing conditions is consistentwith this deduction. Raman normalised photoluminescence spectra recordedat 77 K with 514 nm excitation before and after annealing indicated thatthe annealing treatment had caused a decrease in photoluminescence fromnitrogen-vacancy centres and this coupled with a decrease in theassociated absorption may have contributed to the change in perceivedcolour.

EXAMPLE 3

[0122] A 3.10 mm thick CVD layer was grown on an HPHT synthetic diamondsubstrate in a process similar to that set out in Example 1. The singlesubstitutional nitrogen concentration in this sample was estimated to beapproximately 0.5 ppm from the strength of the 270 nm feature in theabsorption spectrum. The absorption spectrum also contained broad bandscentred at approximately 360 nm and 515 nm and a general rise (ramp) inabsorption coefficient from the red to the ultra-violet.

[0123] The layer was polished into a round brilliant of 0.51 carats thatwas graded as light brown, 13. It was then annealed at 1700° C. fortwenty-four hours under diamond stabilising pressure of approximately6.5×10⁹ Pa (65 kbar). Without any further processing it was graded aslight orangish pink, I3.

[0124] The culet of the round brilliant was then enlarged to allow anabsorption spectrum to be recorded. This indicated no significant changein concentration of single substitutional nitrogen. The strength of theband at 360 nm and the ramp in the absorption spectrum (from red toultra-violet) had been substantially reduced but the band atapproximately 515 nm remained largely unchanged. The photoluminescencespectrum was dominated by photoluminescence from the N-V defects withzero-phonon lines at 575 nm and 637 nm. Raman normalisedphotoluminescence spectra recorded at 77 K with 514 nm excitation beforeand after annealing indicated that the strength of the N-V luminescencehad not been greatly affected by the annealing treatment.

[0125] The change in perceived colour was predominantly a result of thechange in the absorption spectrum.

EXAMPLE 4

[0126] Single crystal CVD diamond was grown to a thickness of 2 mm on a{100} diamond substrate in a process similar to that set out inExample 1. The gas mixture included 2.5 ppm of nitrogen. The substratewas removed and a polished CVD sample Ex-4 measuring 4.5 mm×4.0 mm×2 mmwas produced.

[0127] This sample had a brown colour. Its UV/visible absorptionspectrum is labelled (a) in FIG. 1. In addition to absorption featuresassociated with single substitutional nitrogen, the spectrum containsbroad bands at approximately 515 nm and 365 nm. There is also a generalincrease in absorption coefficient towards shorter wavelengths.

[0128] The CVD diamond sample was then annealed at 2400° C. for 4 hoursunder diamond stabilising pressure of approximately 8.0×10⁹ Pa (80kbar). After this treatment, it was near-colourless. Its UV/visibleabsorption spectrum is labelled (b) in FIG. 1. The remaining absorptionfits the shape of a type Ib spectrum containing approximately 1.1 ppm ofnitrogen in single substitutional sites. The annealing treatment removedthe additional absorption shown by the as-grown sample.

[0129] From the absorbance spectra of this sample measured before andafter annealing the CIELAB coordinates of the diamond were derived inthe way discussed earlier. They are tabulated below. The annealingprocess greatly reduced the b* coordinate and the saturation, whileincreasing the lightness. Before annealing After annealing a* 2.8 −0.9b* 12.0 1.9 C* 12.3 2.1 L* 72 86

EXAMPLE 5

[0130] Single crystal CVD diamond was grown to a thickness of 3 mm on a{100} diamond substrate in a process similar to that set out inExample 1. The pressure was 250×10² Pa, the substrate temperature 815°C., and the gas mixture contained 7.5 ppm of nitrogen. The substrate wasremoved and a polished cross-sectional CVD diamond slice Ex-5 measuring3 mm×2 mm×0.86 mm was produced.

[0131] This sample had an orangish brown colour. Its UV/visibleabsorption spectrum is labelled (a) in FIG. 2. In addition to absorptionfeatures associated with single substitutional nitrogen, it containsbroad bands at approximately 515 nm and 365 nm. There is also a generalincrease in absorption coefficient towards shorter wavelengths.

[0132] The sample was then annealed at 1900° C. for 4 hours underdiamond stabilising pressure of approximately 7.0×10⁹ Pa (70 kbar).After this treatment, it was near-colourless. Its UV/visible absorptionspectrum is labelled (b) in FIG. 2. The remaining absorption fitsreasonably well to the shape of a type Ib spectrum containingapproximately 2.2 ppm of nitrogen in single substitutional sites. Theannealing treatment therefore removed the additional absorption shown bythe as-grown sample and made it near-colourless.

[0133] From the absorbance spectra of this sample measured before andafter annealing the CIELAB coordinates of the diamond were derived inthe way discussed earlier. They are tabulated below. The annealingprocess greatly reduced the b* coordinate and the saturation, whileincreasing the lightness. Before annealing After annealing a* 4.6 −0.5b* 16.8 3.0 C* 17.4 3.0 L* 58.9 87

EXAMPLE 6

[0134] Single crystal CVD diamond was grown to a thickness of 1.8 mm ona {100} diamond substrate in a process similar to that set out inExample 1. The pressure was 257×10² Pa, the substrate temperature 812°C., and the gas mixture contained 3.8 ppm of nitrogen. The substrate wasremoved and the UV/visible absorption spectrum (labelled (a) in FIG. 3)of the resulting brown diamond plate Ex-6 was measured.

[0135] The sample was then annealed at 1600° C. under diamondstabilising pressure of approximately 6.5×10⁹ Pa (65 kbar) for 4 hours.After this treatment, the dominant component of its colour was pink. TheUV/visible absorption spectrum of the annealed sample Ex-6 is labelled(b) in FIG. 3. This spectrum is made up of absorption relating to singlesubstitutional nitrogen with a concentration of approximately 1.2 ppm, aband centred at approximately 515 nm, and some residual absorption inthe ultraviolet. The annealing treatment has removed the band centred atapproximately 365 nm and has significantly reduced the general rise inabsorption towards shorter wavelengths.

[0136] Raman/photoluminescence spectra with 785 nm laser excitation wererecorded at room temperature, before and after the annealing treatment,using a Renishaw Ramanascope with a CCD detector and fitted with aOlympus BH-2 microscope (×10 objective). It was found that the annealingtreatment had introduced a series of photoluminescence lines in thenear-infra-red region of the spectrum. These included a line atapproximately 851 nm and two broader lines at approximately 816 and 825nm.

[0137] From the absorbance spectra of this sample measured before andafter annealing the CIELAB coordinates of the diamond were derived inthe way discussed earlier. They are tabulated below. The dependence ofthe hue, saturation and lightness on optical path length was modelled inthe way described earlier to give an understanding of the colours thatcan be achieved with different path lengths of the as-grown and annealeddiamond. The results are shown in FIGS. 4 and 5, with those for theas-grown and annealed diamond labelled (a) and (b) respectively. Alsoshown on spectra (a) of FIG. 4 is the means by which the hue angel(h_(ab)) is measured, for an arbitrary point (a*,b*) on the curve.Before annealing After annealing a* 4.0 4.4 b* 14.5 4.8 C* 15.0 6.5 L*72 81 Hue angle 75 47 (degrees)

EXAMPLE 7

[0138] A 2.84 mm thick layer of CVD diamond was grown on a type Ib HPHTsynthetic diamond substrate in a process similar to that set out inExample 1. Growth conditions consisted of 42/25/600 sccm (standard cubiccentimetre per second) of CH₄/Ar/H₂ at 330×10² Pa and a substratetemperature of 880° C. with 24 ppm added N₂.

[0139] The substrate was removed and resulting CVD layer was polishedinto a rectangular cut CVD gemstone of 1.04 carats which was graded by aprofessional diamond grader to have a fancy dark orangey brown colourand a quality grade of SI1.

[0140] The gemstone was annealed at 1600° C. for four hours underdiamond stabilising pressure of approximately 6.5×10⁹ Pa (65 kbar).After this annealing treatment, the gemstone was graded again by thesame diamond grader who judged it to have a fancy intense brownish pinkcolour and a quality grade of SI1.

EXAMPLE 8

[0141] A 1.3 mm thick CVD layer with a very dark brown colour was grownon a {100} HPHT synthetic substrate in a process similar to that set outin Example 1.

[0142] Growth conditions consisted of 30/25/300 sccm (standard cubiccentimetre per second) of CH₄/Ar/H₂ at 330×10² Pa and a substratetemperature of 780° C. with 46 ppm added ¹⁵N₂. The nitrogen isotope usedwas ¹⁵N, which may have displaced lines associated with defectscontaining N from the values normally obtained with ¹⁴N because of themass effect. The substrate was removed and polished slices for annealingexperiments were produced from the CVD layer.

[0143] The treatment conditions are listed below. Slice Temperature (°C.) Time Pressure Final colour 1 1800 4 hours 6.5 × 10⁹ Pa Greenish 21700 4 hours 6.5 × 10⁹ Pa Orangish pink 3 1500 4 hours AtmosphericOrangish brown 4 1400 4 hours Atmospheric Brown 5 1200 4 hoursAtmospheric Brown

[0144] All of the annealed slices, even those annealed at atmosphericpressure (in argon) at 1200° C., 1400° C. and 1500° C., showedsignificant increases in transmission in the visible region of thespectrum, with a corresponding increase in their lightness. Theabsorption spectra of the samples showed various lines as detailed intables below. In the first table, the descriptions (strong, medium,weak, very weak) give an approximate idea of relative sizes of featuresin the spectrum. Where no description is given, the relevant feature wasnot observed. In the second table, “Yes” indicates that the relevantfeature was observed.

[0145] From these tables it can be seen that diamond that has been grownand annealed by the method of this invention can show absorption linesnot seen in as-grown CVD diamond. Many of these lines have not been seenpreviously for diamond produced in any other way and appear to be uniqueto diamond produced by the method of this invention. The most obviousexamples are marked with asterixes in the tables. Many of these linescan also be seen in photoluminescence. As-grown 1200° C. 1400° C. 1500°C. 1700° C. 1800° C. Wavelength (nm)  270 nm band Strong Strong StrongStrong Strong Strong  365 nm band Strong Strong  503 Weak Weak Weak Weak 510 nm band Medium Medium Medium Medium Medium Medium  512 V. weakMedium Weak  553 Weak Strong Medium V. weak  597 Medium Medium V. weak 624 Medium Medium  637 Weak Weak Medium Weak Medium V. weak  667* V.weak Medium Weak Weak V. weak  684* Weak Weak Weak V. weak  713 V. weakMedium Medium Medium Weak  781* Medium Medium Medium Weak  851* V. weakWeak Medium Strong Strong 1000 band Strong Strong Strong Strong StrongMedium 1263* Medium Medium Medium 1281* Medium Medium Medium 1359 StrongStrong Strong Medium Weak Weak 1556 Medium Medium Medium Weak Weak V.weak Wavenumbers (cm⁻¹) 1332 Yes Yes Yes Yes Yes Yes 1340* Yes Yes YesYes 1344 Yes Yes Yes Yes Yes Yes 1353 Yes Yes Yes Yes Yes Yes 1362 YesYes Yes Yes Yes Yes 1371 Yes Yes Yes Yes Yes Yes 1374* Yes Yes 1378* YesYes 1384* Yes Yes 1396* Yes Yes Yes 1405 Yes Yes 1453* Yes Yes 2695 YesYes Yes Yes Yes Yes 2727 Yes Yes Yes Yes Yes Yes 2807 Yes Yes Yes YesYes Yes General CH Yes Yes Yes Yes Yes Yes modes 2949 Yes Yes Yes YesYes 3031 Yes Yes Yes Yes Yes 3054 Yes Yes Yes Yes Yes 3107 Yes 3124 YesYes Yes Yes Yes Yes 3310* Yes Yes Yes 3323 Yes 4677* Yes Yes Yes

We claim:
 1. A method of producing single crystal CVD diamond of adesired colour includes the steps of providing single crystal CVDdiamond which is coloured and heat treating the diamond under conditionssuitable to produce the desired colour.
 2. A method according to claim 1wherein the single crystal CVD diamond is in the form of a layer.
 3. Amethod according to claim 2 wherein the layer of single crystal CVDdiamond has a thickness of greater than 1 mm.
 4. A method according toclaim 3 wherein the single crystal CVD diamond layer has a uniformcrystal quality through its thickness.
 5. A method according to claim 2wherein the single crystal CVD diamond is in the form of a piece of thelayer.
 6. A method according to claim 5 wherein the piece of singlecrystal CVD diamond is a gemstone.
 7. A method according to claim 6wherein the gemstone has three orthogonal dimensions, each dimensionexceeding 1 mm.
 8. A method according to claim 1 wherein the singlecrystal CVD diamond has a concentration of nitrogen in the solid diamondof 0.05-50 ppm.
 9. A method according to claim 8 wherein the lower limitof the range is 0.1 ppm.
 10. A method according to claim 8 wherein thelower limit of the range is 0.2 ppm.
 11. A method according to claim 8wherein the lower limit of the range is 0.3 ppm.
 12. A method accordingto claim 8 wherein the upper limit of the range is 30 ppm.
 13. A methodaccording to claim 8 wherein the upper limit of the range is 20 ppm. 14.A method according to claim 8 wherein the upper limit of the range is 10ppm.
 15. A method according to claim 1 wherein the CVD diamond isproduced in a synthesis method using a gas phase in which the nitrogenconcentration is in the range 0.5 ppm-500 ppm.
 16. A method according toclaim 15 wherein the nitrogen concentration in the gas phase is in therange 1 ppm-100 ppm.
 17. A method according to claim 15 wherein thenitrogen concentration in the gas phase is in the range 2 ppm-30 ppm.18. A method according to claim 1 in which the dominant colour of theCVD diamond after heat treatment is other than brown.
 19. A methodaccording to claim 1 in which the colour of the CVD diamond after heattreatment is in the range pink-green.
 20. A method according to claim 19in which the colour of the CVD diamond after heat treatment is pink 21.A method according to claim 19 in which the colour of the CVD diamondafter heat treatment is fancy pink
 22. A method according to claim 19 inwhich the colour of the CVD diamond after heat treatment is green.
 23. Amethod according to claim 19 in which the colour of the CVD diamondafter heat treatment is fancy green.
 24. A method according to claim 1in which the CVD diamond after heat treatment is near colourless
 25. Amethod according to claim 1 in which the CVD diamond after heattreatment is colourless.
 26. A method according to claim 1 wherein thehue angle of the CVD diamond after heat treatment is less than 65°. 27.A method according to claim 1 wherein the hue angle of the CVD diamondafter annealing is less than 60°.
 28. A method according to claim 1wherein the hue angle of the CVD diamond after annealing is less than55°.
 29. A method according to claim 1 wherein the hue angle of the CVDdiamond after annealing is less than 50°.
 30. A method according toclaim 1 wherein a 1 mm thick parallel-sided layer produced from thediamond after heat treatment has a CIE Lab b* co-ordinate which lies inthe range 0≦b*≦8.
 31. A method according to claim 1 wherein a 1 mm thickparallel-sided layer produced from the diamond after heat treatment hasa CIE Lab b* co-ordinate which lies in the range 0≦b*≦4.
 32. A methodaccording to claim 1 wherein a 1 mm thick parallel-sided layer producedfrom the diamond after heat treatment has a CIE Lab b* co-ordinate whichlies in the range 0≦b*≦2.
 33. A method according to claim 1 wherein a 1mm thick parallel-sided layer produced from the diamond after heattreatment has a CIE Lab b* co-ordinate which lies in the range 0≦b*≦1.34. A method according to claim 1 wherein a 1 mm thick parallel-sidedlayer produced from the diamond after heat treatment has saturation (C*)which is less than
 10. 35. A method according to claim 1 wherein a 1 mmthick parallel-sided layer produced from the diamond after heattreatment has saturation (C*) which is less than
 5. 36. A methodaccording to claim 1 wherein a 1 mm thick parallel-sided layer producedfrom the diamond after heat treatment has saturation (C*) which is lessthan
 2. 37. A method according to claim 1 wherein the heat treatment iscarried out under conditions suitable to increase, modify, reduce orremove absorption bands or other components that contribute to thecolour.
 38. A method according to claim 1 wherein the heat treatment iscarried out under conditions suitable to reduce the concentration ofdefects that cause absorption over wide regions of the spectrum.
 39. Amethod according to claim 1 wherein the single crystal CVD diamond hasan absorption band centred at about 350 nm and the heat treatment iscarried out under conditions suitable to alter the absorption band in away that the colour of the diamond is enhanced.
 40. A method accordingto claim 1 wherein the single crystal CVD diamond has an absorption bandcentred at about 510 nm and the heat treatment is carried out underconditions suitable to alter the absorption band in a way that thecolour of the diamond is enhanced.
 41. A method according to claim 39wherein the alteration of the absorption band is the reduction orremoval of the band.
 42. A method according to claim 1 wherein thesingle crystal CVD diamond has a band, centred in the near-infrared,that extends into the red region of the visible spectrum and the heattreatment is carried out under conditions suitable to alter theabsorption band in a way that the colour of the diamond is enhanced. 43.A method according to claim 42 wherein the alteration of the absorptionband is to decrease or increase the strength of the band.
 44. A methodaccording to claim 1 wherein the heat treatment is carried out in atemperature range of 1200° C.-2500° C. under diamond stabilisingpressure or in an inert or stabilising atmosphere.
 45. A methodaccording to claim 44 wherein the heat treatment takes place at atemperature of at least 1600° C. under diamond stabilising pressure. 46.A method according to claim 45 wherein the heat treatment takes place ata temperature between 1600-1700° C. under diamond stabilising pressure.47. A method according to claim 44 wherein the heat treatment takesplace at a temperature not exceeding 1900° C. at a pressure in thegraphite stable region in an inert or stabilising atmosphere.
 48. Amethod according to claim 44 wherein the heat treatment takes place at atemperature not exceeding 1800° C. at a pressure in the graphite stableregion in an inert or stabilising atmosphere.
 49. A method according toclaim 44 wherein the heat treatment takes place at a temperature notexceeding 1600° C. at a pressure in the graphite stable region in aninert or stabilising atmosphere.
 50. A method according to claim 47wherein the heat treatment takes place at a temperature exceeding 1400°C. at a pressure in the graphite stable region in an inert orstabilising atmosphere.
 51. A method according to claim 47 wherein theinert atmosphere is argon.
 52. A single crystal diamond layer producedby a method according to claim 1 for use in an optical application. 53.A single crystal diamond layer produced by a method according to claim 1for use as an electromagnetic transmission window.
 54. A single crystalCVD diamond layer having a colour in the range pink-green.
 55. A singlecrystal CVD diamond layer according to claim 54 wherein the colour isfancy pink.
 56. A single crystal CVD diamond layer according to claim 54wherein the colour is fancy green.
 57. A single crystal CVD diamondlayer according to claim 54 which has a thickness of at least 1 mm. 58.A single crystal CVD diamond layer according to claim 54 which has auniform crystal quality through its thickness.
 59. A piece of singlecrystal CVD diamond produced from a layer according to claim
 54. 60. Apiece according to claim 59 which has the shape of a gemstone.
 61. Apiece according to claim 60 which has three orthogonal dimensions, eachof which exceeds 1 mm.